A conventional microdensitometer uses a light source to illuminate an object or specimen being examined. Light from the source is shined through the specimen and detected by an electronic sensor (typically, a photo-multiplier) which in the conventional manner is located quite close to the specimen. This is termed "near-field" examination, in contrast to "far-field" where interferometric effects predominate, as will be explained hereinafter. The intensity of the transmitted light from the specimen is measured electronically and provides an indication of the density of one or several fine details of the specimen (such as a "grain" of photographic film). By making a multitude of individual measurements over the area of the specimen, a statistical analysis of the density and graininess of the specimen is obtained. A fuller discussion of the principles, analysis and evaluation of photographic-type imaging processes is given in the book entitled "Image Science", by Dainty and Shaw, published by Academic Press, 1974, (see particularly pages 39, 285 and 286). Another analysis of microdensitometer results is given in the book "The Theory of the Photographic Process", fourth edition, by T. H. James, published by the Eastman Kodak Co., 1977, (see particularly p. 619).
One limitation of a conventional microdensitometer is its very narrow depth of focus (typically less than a small fraction of a millimeter). This in turn limits such apparatus to examining elements of a specimen lying in a very thin layer. But a specimen, such as a piece of photographic film, may actually have important elements of its structure (including the film substrate) at depths far beyond the focused plane of view of the microdensitometer apparatus. And, of course, the unfocused details of these elements cannot be easily characterized, if at all. Another, and more important limitation of a conventional microdensitometer is the fact that the details of an ultra small element of a specimen, such as a micron size "grain" of photographic film, cannot be distinguished by a conventional microdensitometer which uses optical apertures that are typically several dozens of microns in diameter. The use of high resolution photo-diode detectors does not enhance resolution in a conventional microdensitometer. Even the most advanced state-of-the-art semiconductor photo-diodes, having a diameter of only 3 microns, cannot properly determine the details of an element (as distinguished from merely detecting the presence or absence of the element) less than about 12 microns in diameter. In other words, to adequately characterize in "near-field" examination the details of an ultra small element of a specimen, the element should be large enough in diameter that, for example, four or more photo-detectors can span its diameter.
In addition to the limitations described above, a conventional microdensitometer apparatus has very limited dynamic range and very limited viewing diameter (e.g., 48 microns "spot" size).
It is highly desirable therefore to provide an electro-optical apparatus and method which can detect and characterize the details of ultra fine elements very much smaller in diameter, over a very much larger region of a specimen, with very much greater depth of focus, and with greater dynamic range than apparatus or method according to the prior art.